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ECOLOGY AND EVOLUTION: How Ants Invade

Some plant and animal species become highly invasive when introduced to alien habitats by humans, and much ecological research effort is currently devoted to understanding what confers invasive characteristics on such species. The Argentine ant was introduced to the United States from South America a century ago and has displaced many other species of ant.

Tsutsui et al. have pinpointed a key difference between introduced and native populations of the Argentine ant. The former apparently have passed through a genetic bottleneck and have lower genetic variability (half as many alleles at the seven microsatellite loci sampled) than the ancestral Argentinian populations. The reduction in genetic variation reduces, in turn, the amount of intraspecific aggression among the ants, permitting the existence of much larger supercolonies and thus conferring more invasive potential.

The results also may resolve a longstanding puzzle in kin selection theory: individuals display altruism to (related) nestmates and aggression to (unrelated) conspecifics from other nests. The existence of huge single colonies or networks of connected colonies, like those of the Argentine ant, has been a problem for the theory. However, loss of genetic diversity appears to accommodate unicoloniality within the framework of kin selection.—AMS